1. Field of the Invention
The present invention relates to an exposure control technique for an imaging apparatus such as a digital camera.
2. Description of the Related Art
A camera typically has a viewfinder for capturing an object upon photographing. There are two prominent types of viewfinders, namely an optical type viewfinder and an electronic type viewfinder.
Optical viewfinders include those used for a single-lens reflex camera in which light having passed through a photographic optical lens is bent with a mirror and those in which an object is viewed through a viewfinder window without using a photographic optical lens.
Electronic viewfinders are typically used for the recently common compact digital still cameras. More specifically, in electronic viewfinders, an image sensor, such as a charge-coupled device (CCD), to be used when photographing an object receives light having passed through a photographic lens and photoelectrically converts the light into an image signal to display the image signal on an image display device, such as a liquid crystal device (LCD), in sequence. With such a configuration, since the same image sensor can be used both upon viewing and photographing, a finder coverage of 100% can be achieved with ease. Such a configuration of the electronic viewfinder has a greater advantage in finder coverage than that of the optical viewfinder.
The image sensor is applicable not only to a still image photographing and a viewfinder but also to the other purposes. Examples of the other purposes include auto focus (AF) control, in which a focus adjustment to an object is performed automatically, and auto exposure (AE) control, in which a brightness adjustment of the object is appropriately controlled. Upon AF control or upon AE control, the image sensor is used as a range-finding sensor or a light metering sensor, respectively. As described above, there may be such a case where the image sensor performs a photographing preparation operation, such as AF control and AE control, while it serves as a viewfinder for a user to monitor an object image.
A camera includes a diaphragm (iris) for adjusting an amount of incident light and a shutter for adjusting an exposure time. A digital camera includes, in addition thereto, a gain control unit for amplifying/attenuating a signal level read from the image sensor after the image sensor has received light. It should be considered that a diaphragm mechanism not only changes an exposure but also changes a depth of field according to an aperture diameter of the diaphragm. In the diaphragm, as the aperture diameter of the diaphragm becomes larger, the depth of field becomes shallower; and as the aperture diameter of the diaphragm becomes smaller, the depth of field becomes deeper. Taking this optical phenomenon into consideration, it is known that a focus can be prevented from shifting according to the field depth upon photographing by shifting the diaphragm to a full-aperture position upon AF control, namely, by adjusting the focus in the field depth shallower than the focus in the field depth upon photographing a still image.
In a camera with an AF function and AE function, a typical release button moves in two stages when the user operates it for photographing an image. In other words, such a typical camera has a configuration that the AF function and AE function are performed to adjust a focus and an exposure for photographing a still image when the user half-presses the release button, whereas a photographing operation starts when the user fully presses the release button. Hereinafter, the half-press of the release button is referred to as an SW1 operation and a full-press of the release button is referred to as an SW2 operation.
When the AF control is performed with the SW1 operation, as described above, the diaphragm is driven to be shifted to a full-aperture position upon AF control. If the diaphragm is preliminarily set to an aperture diameter to be used in photographing a still image after the AF control has been completed, the diaphragm is no longer required to be driven after the SW2 operation. Thus, a shutter release time lag can be shortened. As described above, as a certain amount of time is required for driving the diaphragm because the diaphragm has a mechanical structure, Japanese Patent No. 03817563 addresses this problem.
Since a certain amount of time is required for diaphragm control, if the diaphragm is driven while the user is using the image sensor as a viewfinder, namely, the user is monitoring an object image, a problem arises in that a deteriorated image may be output to an image display device during an operation of the diaphragm. With respect to such a problem, Japanese Patent Application Laid-Open No. 2006-352905 discusses a method of displaying an image obtained before driving the diaphragm without updating an image displayed on the image display device when the diaphragm is being driven. However, this method may not be a suitable solution in using the image sensor as a viewfinder for capturing the movement of an object, because an image updating operation once appears to be stopped.
Also, Japanese Patent Application Laid-open No. 11-112865 discusses a method of controlling a diaphragm not to move under a certain condition so as to avoid showing the image quality deterioration through the viewfinder upon controlling the diaphragm, or to avoid recording the deteriorated image as a moving image. However, this method may cause a problem that a range for following an object luminance value becomes narrower due to the stoppage of the diaphragm, thus causing a deterioration of the AE performance.
As described above, an image sensor for photographing a still image can be used, alone or in combination, for various purposes, such as for a viewfinder, AF control, AE control, or photographing a moving image, and the diaphragm is driven depending on the requirements, such that a good photographed image can be obtained with a good response. However, there is a case where the image quality deterioration occurs due to the drive of the diaphragm while the camera user is capturing the object image by using the image sensor as a viewfinder. In such a case, there is a problem of notable deterioration of an electronic viewfinder performance.